Field of the Invention
The present invention relates generally to a method and apparatus for drilling deviated wellbores, such as in extended reach drilling. In greater detail, the present invention is concerned with rotary drilling of a deviated borehole, and is directed to vibrating the drill string at a suitable frequency and amplitude to reduce the friction of the drill string against the lower side of the borehole and to promote the free movement of the drill string in the borehole.
Extended Reach Drilling is concerned with rotary drilling procedures to drill, log and complete wellbores at significantly greater inclinations and/or over horizontal distances substantially greater than currently being achieved by conventional directional drilling practices. The success of extended reach drilling should benefit mainly offshore drilling projects as platform costs are a major factor in most offshore production operations. Extended reach drilling offers significant potential for (1) developing offshore reservoirs not otherwise considered to be economical, (2) tapping sections or reservoirs presently considered beyond economical or technological reach, (3) accelerating production by longer intervals in the producing formation due to the high angle holes, (4) requiring fewer platforms to develop large reservoirs, (5) providing an alternative for some subsea completions, and (6) drilling under shipping fairways or to other areas presently unreachable.
A number of problems are presented by high angle extended reach directional drilling. In greater particularity, hole inclinations of 60.degree. or greater, combined with long sections of hole or complex wellbore profiles present significant problems which need to be overcome in extended reach drilling. The force of gravity, coefficients of friction, and mud particle settling are the major physical phenomena of concern.
As inclination increases, the available weight from gravity to move the pipe or wireline string down the hole decreases as the cosine of the inclination angle, and the weight lying against the low side of the hole increases as the sine of the inclination angle. The force resisting the movement of the drill string is the product of the apparent coefficient of friction and the sum of the forces pressing the string against the wall. At an apparent coefficient of friction of approximately 0.58 for a common water base mud, drill strings tend to slide into the hole at inclination angles up to approximately 60.degree.. At higher inclination angles, the drill strings will not lower from the force of gravity alone, and must be mechanically pushed or pulled, or alternatively the coefficients of friction can be reduced. Since logging wirelines cannot be pushed, conventional wireline logging is one of the first functions to encounter difficulties in this type of operation. In such cases it becomes very difficult to push pipe or logging tools into the hole, or to obtain weight-on-the-bit from drill collars.
Hole cleaning also becomes more of a problem in high angle bore holes because particles need fall only a few inches to be out of the mud flow stream and to come to rest on the low side of the hole, usually in a flow-shaded area alongside the pipe. This problem is also encountered in substantially vertical wellbores but the problem is much worse in deviated wellbores. In deviated wellbores the drill string tends to lie on the lower side of the wellbore and drill cuttings tend to settle and accumulate along the lower side of the wellbore about the drill string. This condition of having drill cuttings lying along the lower side of the wellbore about the drill string along with the usual filter cake on the wellbore wall presents conditions susceptible for differential sticking of the drill pipe when a porous formation is penetrated that has internal pressures less than the pressures existing in the borehole. This settling of cuttings is particularly significant in the near horizontal holes expected to be drilled in extended reach drilling.
If differential pressure (borehole mud pressure less formation pore pressure) exists opposite a permeable zone in the formation, then conditions are present for the pipe to become differentially wall stuck. The pipe is partially buried and bedded into a mass of solids, and can be hydraulically sealed to such an extent that there is a substantial pressure difference in the interface of the pipe and the wall and the space in the open borehole. This hydraulic seal provides an area on the pipe for the pressure differential to force the pipe hard against the wall. The frictional resistance to movement of the pipe against the wall causes the pipe to become immovable, and the pipe is in a state which is commonly referred to as differentially stuck.
Pressure-differential sticking of a drill pipe is also discussed in a paper entitled "Pressure-Differential Sticking of Drill Pipe and How It Can Be Avoided Or Relieved" by W. E. Helmick and A. J. Longley, presented at the Spring Meeting of the Pacific Coast District, Division of Production, Los Angeles, Calif., in May 1957. This paper states that the theory of pressure-differential sticking was first suggested when it was noted that spotting of oil would free pipe that had stuck while remaining motionless opposite a permeable bed. This was particularly noticeable in a field wherein a depleted zone at 4300 feet with the pressure gradient of 0.035 psi per foot was penetrated by directional holes with mud having hydrostatic gradients of 0.52 psi per foot. In view thereof, it was concluded that the drill collars lay against the filter cake on the low side of the hole, and that the pressure differential acted against the area of the pipe in contact with the isolated cake with sufficient force that a direct pull could not effect release. This paper notes that methods of effecting the release of such a pipe include the use of spotting oil to wet the pipe, thereby relieving the differential pressure, or the step of washing with water to lower the pressure differential by reducing the hydrostatic head. Field application of the principles found in a study discussed in this paper demonstrate that the best manner for dealing with differential sticking is to prevent it by the use of drill collar stabilizers or, more importantly, by intentionally shortening the intervals of time when pipe is at rest opposite permeable formations.
Brooks U.S. Pat. No. 3,235,014 describes a surface-mounted vibratory type apparatus which may be used with conventional rotary equipment for the drilling of boreholes. The system herein employs a novel form of swivel which causes a kelly, as it is turned by the rotary table, to be vibrated longitudinally and thereby provide combined rotary and vibratory drilling action to a drill string. This swivel can be designed to impart vibrations of desired amplitude and frequency to the kelly and attached drill string. However, the teachings of this patent are not at all concerned with problems of promoting the movement of a drill string in deviated holes, such as are encountered in extended reach drilling, or with mitigating pressure-differential sticking of a drill string in a deviated wellbore.
Solum U.S. Pat. No. 3,557,875 is directed to apparatus for vibrating a well casing through manipulation of the drill pipe during a procedure wherein such vibration is desired. The device therein is adapted to be mounted on a drill pipe and inserted in a well casing, and includes a radially movable impact member resiliently urged into engagement with the well casing. It is repeatedly moved away from engagement and released to cause an impact upon rotation of the drill pipe while the device is resiliently held from rotating relative to the casing. A method is disclosed of cementing or gravel packing the casing in the well by the use of such devices to vibrate the casing while the cement slurry or gravel is pumped through the drill pipe and into the annulus surrounding the casing. A further example therein of where such vibration is desired is in the running of casing in a slant well wherein in the wellbore starts out vertically, is deviated, and then by a second deviation is returned to the vertical or to a slightly inclined direction. A plurality of casing impacting and vibrating devices are mounted in spaced relation along a length of casing attached to the drill pipe and within a larger diameter casing. Rotation of the drill pipe actuates the devices, and vibrates the larger casing. This patent is also not concerned with problems of promoting the movement of a drill string in deviated holes, such as are encounted in extended reach drilling, or with mitigating pressure-differential sticking of a drill string in a deviated well bore such as in extended reach drilling.